[FIX] HeatMap: Color gradient center value edit

[ales-erjavec]

Issue

Fixes gh-5640

Description of changes

Replace use of plain line edit with a DoubleSpinBox

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5647 (adc9fbb) into master (0a34127) will decrease coverage by 0.00%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master    #5647      +/-   ##
==========================================
- Coverage   85.97%   85.97%   -0.01%     
==========================================
  Files         313      313              
  Lines       65640    65675      +35     
==========================================
+ Hits        56436    56464      +28     
- Misses       9204     9211       +7     

[robertcv]

Works for me although I have to use a comma as the delimiter. I am using the Slovenian setting on my computer. Is the use of system language-specific delimiters applied everywhere in Orange? I thought it was English-specific everywhere.

[markotoplak]

Thanks for bringing a spinbox with non-fixed decimal precision/rounding to my attention: I did not know Orange has it.